The present invention relates to a densitometer for measuring the density value of a to-be-measured substance such as a solid or suspension matter in a to-be-measured liquid using microwaves.
A densitometer using microwaves measures density by measuring the phase delay of microwaves on the basis of the fact that the microwaves have a phase delay almost proportional to the density value of a to-be-measured substance in a to-be-measured liquid as a medium.
A densitometer of this type using microwaves comprises transmission and reception applicators 62 and 64 arranged in a tube 63 as microwave antennas, a microwave circuit 79 as an electronic circuit, and a calculating section 81, as shown in FIG. 1A. An oscillator 55 generates microwave signals 56 and 57 having a frequency f. The microwave signal 56 is amplified by an amplifier 58. When switches 59 and 60 are in the states shown in FIG. 1A, a transmission signal 61 is sent to the transmission applicator 62 in the tube 63 and then into the tube 63 in which a liquid 63A to be measured is passed. The signal is received by the reception applicator 64 which is set in the tube 63 to oppose the transmission applicator 62.
A reference oscillator 65 generates reference signals 66 and 67 having a frequency f+.DELTA.f slightly different from the frequency f of the microwave signals 56 and 57 from the oscillator 55. The microwave signal 57 and reference signal 66 are mixed by a mixer 68 to obtain a reference-side heterodyne output 69 as a difference frequency .DELTA.f. The output 69 is converted into a reference-side digital signal .theta.REF 71 through a low-pass filter 69A and a comparator 70 whose threshold value is 0V, and sent to a phase difference measuring section 72.
A reception signal 73 received by the reception applicator 64 is amplified by an amplifier 74. The amplified reception signal 73 and the reference signal 67 are mixed by a mixer 75 to obtain a measurement-side heterodyne output 76 as a difference frequency .DELTA.f. The output 76 is supplied to a comparator 77 via a low-pass filter 76A, converted into a measurement-side digital signal .theta.FB 78, and sent to the phase difference measuring section 72.
The phase difference measuring section 72 obtains a phase difference .PHI.V between the two digital outputs .theta.REF 71 and .theta.FB 78. As shown in FIG. 1B, the difference between the leading edges of the signals .theta.REF and .theta.FB is obtained as the phase difference .PHI.V.
In the microwave circuit 79 indicated by the dotted line, the phase changes due to, e.g., a change in temperature in the circuit to generate an error. To compensate for the error, the switches 59 and 60 are connected to the sides opposite to those in FIG. 1A to measure a phase difference .PHI.R through a fixed reference 80, and the phase difference .PHI.R is subtracted from the phase difference .PHI.V.
The fixed reference 80 uses an attenuator to drop the signal level of the microwave to the same level as that of the signal received by the applicator 64.
A phase difference .PHI. is given by .PHI.=.PHI.V-.PHI.R.
When data (calibration curve data) associated with the reference density is obtained in advance, a calculating section 81 can calculate, on the basis of the data, the density value of the to-be-measured substance in the to-be-measured liquid 63A from the obtained phase difference .PHI..
Let D be the density value. The relationship between the density value D and the phase difference is essentially described by linear equation (1). Values a and b can be determined by measuring phase differences while changing the density value and performing regression analysis. EQU D=a.PHI.+b . . . (1)
In water as a conductive fluid to be measured, the following relationship holds between the attenuation and phase delay of a microwave, and a conductivity a, permittivity, and temperature t of the fluid to be measured: ##EQU1##
where .sigma. is the conductivity of the fluid to be measured, and .epsilon.r' and .epsilon.r" are respectively the real part and imaginary part of the complex relative permittivity of the fluid to be measured.
As is well known, when the density of sludge or pulp as a substance to be measured changes, the effective permittivity changes accordingly. Especially, the permittivity real portion and the density value have high correlation.
When ##EQU2##
in equations (2) and (3), the permittivity imaginary portion is small. When the conductivity is also low, we obtain ##EQU3##
The attenuation amount and phase delay are obtained from .alpha. and .beta. obtained on the basis of equations (4) and (5). Let P0 be the transmission power, and P be the microwave power traveling in the z direction. Then, EQU P=P0exp (-2.alpha.z) (6)
The attenuation amount is 20 .alpha.z/ln10 dB.
The phase delay is .beta.z rad.
In the above-described scheme, the density value is obtained by obtaining the phase delay. As shown in equation (5), .epsilon.r' is in proportion to .beta. in the small change range of .epsilon.r'. For this reason, the density value is obtained from .beta.z. Since .alpha. has lower correlation than .beta., .alpha. is not directly used for density measurement.
The above-described conventional densitometer using microwaves has the following problems.
(a) When the temperature or conductivity of the liquid to be measured changes, the amount of attenuation of a microwave due to the liquid to be measured greatly changes. When the microwave attenuates to decrease the amplitude of the measurement-side heterodyne output 76, the switching time for phase difference measurement changes due to the influence of noise or drift in digitizing the reception signal with the comparator 77, resulting in a measurement error.
(b) Due to the same reason as in (a), when power of the reception signal 73 changes, the phase changes due to the non-linearity of the electronic circuit, resulting in a measurement error.
(c) The influence of temperature drift in the electronic circuit is compensated for by the fixed reference attenuator. When the signal level of the measurement-side heterodyne output 76 changes, the influence of temperature changes and therefore cannot be completely compensated for.
(d) The density value is obtained on the basis of a change in phase. For this reason, when the phase of the reception signal 73 exceeds 360.degree., the density value cannot be accurately obtained.
More specifically, when the tube diameter is large, or the substance to be measured has a high density, the phase changes by 360.degree. or more, and the density value cannot be uniquely determined from the change in phase. In continuous measurement, the number of cycles can be estimated from previous/next measurements, as disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 8-82606. However, when the tube empties and is filled with the liquid to be measured again, no accurate density value can be measured.
(e) The microwave is received through portions other than the liquid to be measured because of runaround or induction from the wiring pattern of the circuit, resulting in a measurement error.
(f) When bubbles are present in the liquid to be measured, the microwave traveling path becomes long, or the microwave is reflected and received through a plurality of paths, resulting in a measurement error.
(g) When the temperature or conductivity of the liquid to be measured changes, the phase of the microwave changes to generate an error. To prevent this, the temperature or conductivity need be measured and corrected. A method of measuring conductivity has been proposed, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-43181. However, a conductivity sensor is readily contaminated. This decreases the measurement accuracy or poses a problem of maintenance. Hence, this method can hardly be put into practical use.
(h) The microwave circuit is expensive and increases the cost of the device itself.